1. Field of the Invention
The present invention relates generally to a fail-safe system and brake actuating apparatus for a vehicular brake control apparatus, and particularly to a fail-safe system for a vehicular brake control apparatus having both an electric brake system so-called "by wire" type brake apparatus and an auxiliary mechanical brake system, capable of immediately actuating the mechanical brake system even when an unexpected fault was occurred in the electrical brake apparatus. More particularly, the invention relates to a brake actuating apparatus to which the above fail-safe system is applied.
2. Related Art
A conventional power brake control apparatus in which electric signals are employed for actuating the brake system is disclosed, for example, in U.S. Pat. No. 4,802,562 and Unexamined Japanese Patent Application (OPI) No. Hei. 2-267056. In these conventional brake apparatus of the by-wire type, a depressing force applied to or stroke of a brake pedal is detected by a sensor, and an electric control device controls a brake actuating apparatus in accordance with an output signal of the sensor, so that wheel cylinders disposed at vehicle wheels are supplied with a desired brake pressure.
The by-wire system of the brake apparatus as described, above is in fact advantageous in that it is readily applied to an anti-skid brake system and/or traction control system since the electric signals are applicable to various kinds of controls. Further, since the brake force control structure is relatively simple compared to the other brake control apparatus, the demand for this type of brake control apparatus will likely increase in future.
However, this type of brake control apparatus suffers from a problem that control is lost when an unexpected electrical defect occurs due to a fault of the sensor or a breakdown of electric wires, since the by-wire system brake apparatus electrically controls the whole the operational force for the brake control apparatus in accordance with the electric signal supplied from the sensor detecting the depressing force applied to or stroke of the brake pedal. This is different from the former conventional mechanical brake operating structure which does not depend on electrical signals. Accordingly, the conventional electrical brake actuating apparatus raises a serious problem in safety.
Further, in the brake actuating apparatus to which the by-wire type brake apparatus is applied, as disclosed in the U.S. Pat. No. 4,802,562 and Unexamined Japanese Patent Application (OPI) No. Hei. 2-267056, a spool valve or the like performs an initial operation and then the other piston is actuated to generate brake pressure. Accordingly, the structure is quite intricate.